With the expansion of information processing technology, large amounts of diverse text data are now analyzed for new findings in a wide variety of fields. Examples include the analysis of microblogging data over networks, product information data at manufacturers, product sales data at sellers, and clinical data at medical institutions. When processing text data, various approaches, including those of the present inventors (Patent Literature 1 and 2) have been proposed for the so-called top-k problem, which is extracting the top k among frequently occurring words. A neighborhood search technique has also been proposed which uses an inverted index to determine which words occur frequently in the neighborhood of a given keyword (Patent Literature 3 through 14).